Field of the Invention
The invention pertains to apparatus and methods for heat treating coatings. The invention more particularly pertains to apparatus and methods for uniform heat treatment of these coatings on glass, ceramic, and/or flexible substrates using microwave energy.
Description of Related Art
There are several methods for depositing thin films on glass substrates and the number of available methods continues to expand with the growth of the flat panels market.
Some techniques require vacuum and thermal evaporation, whereby the material is heated and it evaporates to deposit a thin film on the substrates. These include vacuum evaporation deposition, Molecular Beam Epitaxy (MBE), different variants of Chemical Vapor Deposition (CVD) and Atomic Layer Epitaxy (ALE), studied extensively for semiconductor deposition and electroluminescent display applications, lately also referred to as Atomic Layer Deposition (ALD).
Another technique “Vapor Phase Deposition” (VPD) is proven to give better control on the structure and morphology of the film than vacuum thermal evaporation. It is usually applied to organic materials and hence also termed as Organic PVD or OPVD. The process involves evaporation of the organic material (hence lower temperatures) over a substrate in the presence of an inert carrier gas (N2, argon, H2 and forming gas). For more monomers, the monomer precursors can be co-deposited followed by a polymerization heat treatment.
Most films deposited on substrates need a heat treatment. Organic materials need to be cured well to provide optimum dielectric properties. Coatings for thin film transistors (TFT) in semiconductor electronics, flat panel displays (FPD) and thin film photovoltaic cells, all need the thermal treatment. Although some heat treatments could be carried out during the deposition process or in the same chamber, but usually the cures or anneals are time consuming and hence the thermal treatment is performed in a following separate step. Thus, the thermal treatment is conducted in curing ovens, on hot plates, in annealing furnaces, and of course with microwave energy as well.
Microwave energy because of its rapid and internal heating mechanism has been one of the very attractive means of heat treatment. For curing processes microwaves interact with the polar groups of molecules in the organic materials, enhance their mobility because of the rotational movement of molecules and hence enhance the cross-linking of monomers or cure of materials. In other semiconductor materials or coatings used for thin film transistors (in electronics, flat panel displays and photovoltaic applications), the microwave induced transport of current carriers and possible polarization, the anneal processes can be performed in shorter times or at somewhat lower temperatures. Although fixed frequency microwaves can be used for some applications, the heating across the large substrate surface may not be uniform. When the substrates have electrical traces and electronic circuits there is always the potential of charge build up on metal features which can lead to arcing and hence damage of the circuit.
In such situations Variable Frequency Microwaves (VFM) is well suited for uniformly processing electronic circuits and semiconductor materials. The basic VFM approach is well-known and taught in several U.S. Patents. The continuous sweeping of frequencies over the available bandwidth reduces the potential for arcing and subsequent damage. Frequency sweeping is carried out in a substantially continuous way over a selected bandwidth of frequencies. This provides much more uniform heating without the concerns of charge build up and arcing observed with single frequencies. Numerous kinds of wafers with integrated circuits have been exposed to VFM and it has been demonstrated that there is no damage to the circuits or their functionality.
However, for most of the coatings on the flat panels, the amount of material deposited as a thin film is a small fraction of the mass of the substrate, which is usually glass. In many cases microwaves will not heat the substrate significantly, so any heat generated in the coating material will be lost to the substrate acting as a huge heat sink. Thus the overall increase in temperature on the film will not be sufficient to perform the thermal treatment on the deposited film.